1. Field of the Invention
This invention relates to a process for electrophoresis, medium employable therefor and a support for preparing and/or supporting the medium.
2. Description of Prior Arts
There has been heretofore known electrophoresis for separating proteins, decomposition products of protein, nucleic acids, and decomposition products of nucleic acid by means of a sheet-type medium such as a gel membrane or filter paper impregnated with a buffer solution, which is based on phenomenon that the charged particles of said substances migrate under the influence of the electric field. Particularly, electrophoresis has been advantageously performed for the purposes of separation and identification of biopolymers as mentioned above.
In the genetic engineering field which has been paid more attention recently, the electrophoresis is regarded inevitable for determination of base sequence of nucleic acids such as DNA. Electrophoresis performed for the above purpose generally includes a process of causing a plurality of mixtures of radioactively labeled DNA or mixtures of base specifically cleaved products of DNA to migrate in parallel along the longitudinal direction of a medium for electrophoresis. The base sequence is determined by comparing a plurality of thus obtained electrophoretic patterns (an aggregate of zones (or bands) formed in a medium by electrophoresis.) This process is based on a principle that the base specifically cleaved products having the same molecular weight would migrate to the same positions as each other, as far as the starting positions for the electrophoresis procedure is the same as each other.
In the practical runs of electrophoresis, however, the substances having the same molecular weight are apt to migrate to the different positions. Therefore, the respective migration distances of the substances are not equal. In other words, the migration rate of charged substance is generally apt to be lower at the both side portions than at the central portion, and therefore, the pattern of electrophoresis after a lapse of certain time shows that the migration distance is shorter at both side portions than at the center portion as shown in FIG. 1. FIG. 1 is a schematic view of electrophoresis patterns of zones 13 and 13' obtained by electrophoresis starting from a starting point 12 on a medium 11. This phenomenon is called the "smiling effect". The smiling effect reduces the accuracy of the result obtained by the determination process of base sequence of DNA which involves a procedure of comparing a plurality of rows (i.e., lane) of electrophoresis.
The smiling effect is mainly caused by a difference in temperature between the center portion and side portion which is brought about by escape due to radiation of heat generated in the medium (Joule's heat) from the side edge portions. In more detail, while at the center portion of the medium the generated Joule's heat radiates from the upper and bottom surfaces, at the side edge portions Joule's heat radiates not only from the upper and bottom surfaces but also from the side edge portion. In order to compensate for the heat radiation, heat moves in the lateral (width) direction. The heat radiation can be easily compensated at the center portion of a medium because heat comes in from both sides. On the other hand, heat radiation cannot be fully compensated at the both sides because heat comes in from only one side. For these reasons, the temperature at the side portions is apt to be lower than at the center portion. In order to prevent generation of the smiling effect, a heat radiation plate is generally provided over the surface of an electrophoresis medium so as to reduce the difference of temperature. However, it is difficult to completely prevent the smiling effect by this method. Moreover, it is difficult to adapt the heat radiation plate to the electrophoresis medium.